In other threads it has been mentioned or implied that delay action shells do not detonate prematurely, particulary with refence to the POW shell that pierced Bismarck's forward oil tanks.

What exactly happens when an AP shell hits the surface of the water - does not the impact at that point correspond to contact with a solid object such as the side of a ship and then trigger delay action detonation?

I ask this because I was told some years ago that in diving, particulary high diving, that hitting the water can be like hitting a brick wall - although I should confess I haven't tried it myself so I don't really know. But if the friction of water tension is substantial it would be a trigger? Or is it the shallow angle of contact that dissipates the force of impact?

My understanding was that Japanese "diving" shells at extra long fuzes to account for the time it took them to reach the ship after they hit the water. I guess this could also be due to them going slower when they hit though. Good question I'm looking forward to hereing the answer but suspect it may not be defintive. Ie sometimes yes and sometimes no that may vary in porportion depending on the nationality and date of the fuze to say nothing of impact angle.

My impression is that WWII AP shells activated their fuse when they hit water, just like hitting armor. Then they would go off after the normal delay. Sometimes they failed to explode, just as they did after passing through armor plate. Japanese shells were designed to be hydrodynamically stable and had an extra long fuse delay in an attempt to achieve more damaging underwater hits. The long delay and shell design resulted in them not performing as well when NOT hitting water.

Water will provide enough resistance to slow a shell abruptly so that the graze action fuze in the base initiates--that is the inertial slug has sufficient force to fly forward against its coil spring and fire the detonator, although at very shallow angles of impact, the shell might skip off the surface of the water without the shell decelerating enough for the slug to overcome the tension of the retaining spring. As Bgile said the Japanese incorporated an exceptionally long fuze delay to allow for the long underwater trajectory of the shell. The trade off was, naturally, a shell might score a direct hit against light armour and go straight on through without detonating on board whereas an AP shell with normal delay could.

Their shoulders held the sky suspended;
They stood and Earth's foundations stay;
What God abandoned these defended;
And saved the sum of things for pay.

Coming back to the POW shell penetrating the Bismarck oil tanks, how would the time delay mitigate against a detonation at the critical point of actually passing this infrastructure. If the shell is slowed down then would it not be likely to detonate at around that point, or am I missing something?

I'm waiting for Bill to chime in here. I recall him posting some very insightful views on this very subject several years ago.

Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

RF wrote:Coming back to the POW shell penetrating the Bismarck oil tanks, how would the time delay mitigate against a detonation at the critical point of actually passing this infrastructure. If the shell is slowed down then would it not be likely to detonate at around that point, or am I missing something?

The shell that hit Bismarck's bow apparently didn't encounter any water or enough armor to activate it's fuze. It seems to have simply passed through the ship. The latest photo I've seen of the hit shows it entering just below weather deck level and exiting somewhat lower in keeping with a normal trajectory. The exit hole is much larger, but that would happen if the shell hit something substantial and carried it out of the ship in pieces. I really don't think the shell was slowed much.

The underwater ballistics of spinning projectiles tends to be highly unpredictable. Very few tests were completed, and in those tests which were completed, the results were often problematical. Prior to the advent of modern technologies, about the only way to track a projectile in its underwater trajectory was via photography, and this ended up being entirely impractical due to the the obscuring effects of the impact splash along with the need to photograph the subsequent movement of a bullet that was often twenty or more feet below the surface, in two directions simultaneously, i.e. once from the top (for deflection) and once from the side, (for trajectory curvature). In practical terms, about the best that could be done was to fire the projectile 'short' so that it subsequently passed through a series of nets. This gave a rough idea of the depth of the shell at any point, and its deflection as well, although its orientation and speed often remained unknown.

A lot more testing was done during World War II, but unfortunately -- at least for those interested in gunnery -- most of these tests involved the entry characteristics of torpedoes and rockets, both of which are typically much longer than a spin stabilized projectile, and both of which typically enter the water at well under the velocities normally encountered with projectiles.

That being said, the general plan of the underwater trajectory is fairly well understood. After impact, most spin stabilized projectiles tumble rather violently, primarily because the spin rate necessary to stabilize them in air is several orders of magnitude less that that that would be required to stabilize in water. Often, after a short travel, the projectile restabilizes in a nearly 'base-first' attitude, rotated about 150 degrees from its original orientation. The high density of water means that projectile deceleration is also very rapid; as a 'first cut', the velocity at any point can be quickly approximated via standard momentum equations, i.e. by assuming the momentum of the projectile is transferred into displacing a given volume of water. In general, the trajectory underwater will take the form of a nearly circular curve, concave upwards, with the projectile re-exiting again at some point down range from the impact point if it retains enough initial velocity to do so. Curiously, the radius of the underwater trajectory is as well correlated with the relative density of the projectile and the water, than with any characteristics of its external geometry.

As might be expected, fuze action, as has been demonstrated in actual battle damage, appears to be somewhat unreliable. This is probably at least in part due to the relative unpredictability of the post-impact trajectory, and the associated inertial loads etc. expressed upon the fuze train.

Incidentally, the oft-seen and reproduced graphic of the Japanese 'diving shell' striking Tosa definitely does not represent a typical case, even for a shell designed specifically to dive. Although it seems to be a fairly common thought that the Japanese in some way 'invented' this flat-nosed concept, in reality it had been known -- and employed -- rather routinely at least since the turn of the century, with ships being equipped with special flat-nosed 'diving' shells specifically to attack submarines. Projectiles, even diving projectiles, only very rarely travel long distances in a horizontal mode under water. The trick is to balance the upward forces causing the trajectory to curve, with the decreasing velocity and to keep these two 'in balance' so that the projectile neither rises nor falls as it slowly 'runs out of steam'. This is very difficult to do, perhaps practically impossible when one considers that the designer actually has control of only three or four of the eight or ten variables that influence the physics of the situation. It appears most likely that either the Tosa diagram represented an extremely unusual trajectory, or that the Japanese were confused and connected the underwater hit on Tosa to the wrong projectile splash.

I have published approximate equations for the underwater trajectories of spin-stabilized projectiles and will not repeat them here. Keeping in mind that the trajectory to trajectory scatter is typically very large, i.e. that the equations are more precise than accurate, they nevertheless can predict the AVERAGE underwater performance of a projectile quite well.

Bill, you seem to be saying the Japanese designed their AP shells based on incorrect assumptions and suffered the consequences, which was relatively poor performance when the shell hit something above the waterline.

This is really interesting, because there are those who think the US "super heavy" shells used in all our cruisers and battleships were based on incorrect assumptions as well. They don't seem to perform very well in Okun's formulas either, with the british 14" actually seeming to be in the same ballpark wrt deck penetration.

I do not have enough information at hand to do more than speculate regarding the background of Japanese projectile design. I did not mention consequences at all, which is another topic.

One of the characteristics of underwater trajectories is that they are really quite unpredictable unless angle of fall and striking velocity fall within narrow limits, and -- at least for gunnery purposes -- these both vary rather dramatically with range. One can get fairly predictable trajectories with torpedoes, etc., but this is because the aircraft dropping the torpedo can be constrained to fly at a specific height and speed, thus rendering most entry conditions fairly similar from drop to drop. But this doesn't happen in gunnery. In some cases, closed-form solutions -- approximations might be a better word -- to the underwater trajectory problem can be obtained, although these are often only applicable overy fairly narrow ranges of entry and projectile shape. For unusual shapes and odd entry conditions, then the only answer is numerical integration of the underwater trajectory or rather uncertain extrapolation of terms into unknown mathematical territory. In either case, the equation results would almost certainly require physical testing to confirm accuracy as well.

This inherent unpredictabilty of the underwater trajectories of spinning short projectiles is demonstrated in a number of cases. Certainly the oft-reproduced underwater hit on TOSA, if it is correctly depicted in the diagrams, would appear to show a fairly atypical underwater trajectory. So, for that matter, was the underwater hit on Prince of Wales, which went much deeper than normal equations would estimate. Early flat-nosed diving shells, as mentioned before used against submarines or in battle practices where ricochets might be dangerous, were not as much designed to stabilize the underwater trajectory, as to decrease the probability of ricochets, especially when the angle of fall was small, as would normally be the case when a high-velocity gun is firing at a relatively small target quite close aboard, i.e. a submarine.

I do not know any details regarding the structure and result of any Japanese testing program for their diving shells. Several general possibilities exist, viz:
a) They conducted no substantive formal testing program at all, and just took the TOSA results as being typical.

b) that their testing program (if it existed) enabled them to develop a diving projectile which actually was stable underwater and did deliver an 'as-designed' and predictable underwater trajectory, and

c) That their testing program (if it existed) demonstrated a highly-chaotic underwater trajectory, and they just went ahead with the design plans anyway.

Certainly if the Japanese proceeded using the TOSA results without further testing, etc., then their design and development process might best be characterized as 'imprudent'. If they did actually manage to create a reliable diving projectile, then this would be the product of either extremely good luck, or a truly outstanding effort in engineering. I suspect, again without any substantive supporting evidence, that neither situation applied, i.e. that the engineering behind these projectiles was only 'normal', and that they weren't particularly lucky, i.e. that over most tactical conditions the so-called 'diving projectiles' would not perform very well at all.

Bgile wrote:
This is really interesting, because there are those who think the US "super heavy" shells used in all our cruisers and battleships were based on incorrect assumptions as well. They don't seem to perform very well in Okun's formulas either, with the british 14" actually seeming to be in the same ballpark wrt deck penetration.

I think you have it reversed. Okun's formulas, as per NAaB show the USN 16/45/2700lb shell as having quite superior deck penetration, but equivalent belt penetration to the RN 14/45.

Bgile wrote:
This is really interesting, because there are those who think the US "super heavy" shells used in all our cruisers and battleships were based on incorrect assumptions as well. They don't seem to perform very well in Okun's formulas either, with the british 14" actually seeming to be in the same ballpark wrt deck penetration.

I think you have it reversed. Okun's formulas, as per NAaB show the USN 16/45/2700lb shell as having quite superior deck penetration, but equivalent belt penetration to the RN 14/45.

I was referring to the 16"/50 cal gun.

I may be mistaken, but when I was looking at the calculation it appeared that the 2700 lb shell is penalized because it's cap is heavier and the cap's mass and inertia doesn't effect penetration at all. While I realize it gets discarded, all that steel is going to go somewhere, not just vaporize. He doesn't deal with things like the shell body being very thick and less likely to deform or shatter either ... if I am remembering correctly.